NIAMEY/BAMAKO, Jan 22 (Reuters) - Chadian forces advancedtowards the Malian border on Tuesday as an African troopdeployment and a U.S. military airlift swelled internationalsupport for French operations against Islamist rebels occupyingthe north of Mali.

An armoured column of Chadian troops, experienced in desertoperations, moved north from the Niger capital Niamey on theroad to Ouallam, some 100 km (60 miles) from the Malian border,where Nigerien troops are already stationed.

France, which launched air strikes in Mali 11 days ago tohalt a surprise Islamist offensive toward the capital Bamako,has urged a swift deployment of the planned U.N.-mandatedAfrican force to back up its 2,150 soldiers already there.

The number of French troops could be boosted to more than3,000 in the coming days and weeks, a source with knowledge ofthe matter said on Tuesday.

The aim of the intervention is to prevent northern Mali frombecoming a launchpad for international attacks by al Qaeda andits local allies in North and West Africa. Fears of this increased sharply after a hostage-taking raid by Islamistmilitants last week on a gas plant in Algeria.

An entry into Mali from Niger by part of the African forcewould widen the front of operations against the Islamistalliance in the north that groups al Qaeda's North African wingAQIM and the Malian militant groups Ansar Dine and MUJWA.

On Monday, French and Malian armoured columns moved into thetowns of Diabaly and Douentza in central Mali after the rebelswho had seized them fled into the bush to avoid air strikes.Diabaly is only 350 km (220 miles) north of Bamako, whileDouentza is 800 km away from the riverside capital.

Niger President Mahamadou Issoufou, who visited the troopsat Ouallam, condemned the Islamist alliance, and an imam, orMuslim cleric, said prayers for the troops.

"We are going to war. A war imposed on us by traffickers ofall kinds, an unjust war, from which the peaceful citizens ofnorthern Mali are suffering terribly," Issoufou told the forces.

"I am confident in your burning desire for victory."

France says its troops will remain in Mali until they havecompletely dislodged the Islamist fighters from the north andfair elections can be held in its former colony.

In support of France, the United States has startedtransporting French soldiers and equipment to Mali from theIstres air base in southern France. Washington on Tuesdaycompleted the fifth of an estimated 30 flights in an airliftexpected to run for about a week.

A Reuters correspondent in Bamako saw a U.S. military cargoplane land at the international airport and offload about 40French soldiers, jeeps, and other equipment.

Britain, Belgium, Canada and Denmark were alreadytransporting French materiel to Mali. Benson said the UnitedStates was also working with France on intelligence issues, butdeclined to say if surveillance drones were being used.

"EVERYONE WILL FIGHT"

France has also sent jet fighters and attack helicoptersthat have blasted rebel bases for more than a week, as it awaitstroops from nearby African nations to deploy to the front line.

Some 1,000 African troops from the West African regionalbloc ECOWAS and the central African nation of Chad have arrived,and that number is expected to top 5,000 in the coming weeks.

Military experts say the swift and effective deployment ofAfrican forces is crucial to sustain the momentum of France'sair campaign and prevent Islamists from melting away into theempty desert or the rugged mountains near the Algerian border.

Niger's armed forces, which completed their training a monthago, are expected to advance toward the rebel-held north Maliancity of Gao in collaboration with the Chadian troops. It was notclear when exactly they would cross the border.

Gao, the largest city of Mali's north, has been hit byFrench air strikes in recent days.

Niger has already sent a technical team to Mali, part of abattalion of 544 troops accompanied by French liaison officers.

Nigeria, a big oil producer, also plans to deploy some 1,200troops in Mali and its president, Goodluck Jonathan, said theywould stay there until the crisis was resolved.

Colonel Oumar Kande, ECOWAS military and security adviser inMali, told Reuters in Bamako the original plan for theU.N.-backed ECOWAS military intervention in the north was beingchanged to adapt to fast-evolving circumstances.

Instead of the Malian army alone playing the combat role,with ECOWAS supporting, now "everyone will fight", Kande said.